Twist-axle with longitudinally-varying wall thickness

ABSTRACT

The invention relates to a twist-axle that includes a cross-beam member and two trailing arms, each trailing arm rigidly secured to the cross-beam member in one of two connection regions of the cross-beam member or formed integrally with and extending from one of the two connection regions. The cross-beam member is formed from a tubular blank and has a torsionally elastic central portion and two torsionally stiff connection regions. The cross-beam member has a wall thickness that varies longitudinally along the length of the cross-beam member from the torsionally elastic central portion to each of the torsionally stiff connection regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.13/124,509. filed on Apr. 15, 2011, and entitled TWIST-AXLE WITHLONGITUDINALLY-VARYING WALL THICKNESS, which is a Submission Under 35U.S.C. §371 for U.S. National Stage Patent Application of InternationalApplication Number PCT/CA2009/001456, filed Oct. 15, 2009, which isrelated to and claims priority to U.S. Provisional Patent ApplicationSer. No. 61/106,389, filed Oct. 17, 2008, the entirety of both areincorporated herein by reference, and to Canadian Patent ApplicationSerial Number 2,644,464, filed on Nov. 28, 2008, the entirety of bothare incorporated herein by reference.

FIELD OF INVENTION

The invention relates generally to the field of tubular components forsuspension and chassis structures, and has particular application totubular twist-axle of a vehicle.

BACKGROUND OF INVENTION

In a vehicle, a twist-beam or twist-axle is often used as a rearsuspension structure. A twist-axle has two trailing arms for connectingroad wheels to a vehicle's frame and a cross-beam member linking thetrailing arms to form an integral body. Each trailing arm is pivotallyconnected to a vehicle's frame. When the road wheels are unequallydisplaced relative to the vehicle's frame, such as when the wheelsencounter an uneven surface or when the vehicle is turning, the unequaldisplacement causes the trailing arms to pivot by different amounts,thereby resulting in twisting of the cross-beam member. The inherenttorsional stiffness or resistance of the cross-beam member provides arestoring force to the unevenly displaced wheels.

For ride comfort and controllability (i.e., handling) of the vehicle,twist-axles generally need to meet compliance requirements for torsionalstiffness. By compliance of torsional stiffness, it is meant that atwist-axle needs to have a torsional stiffness within a specified range.On the other hand, a twist-axle is a load bearing component and must bedesigned to have sufficient strength to support linear loads, such asstatic weight of a vehicle and dynamic load created as the vehiclemoves.

There have been many proposals to make twist-axles that meet bothtorsional or roll stiffness and load bearing requirements. For example,there have been proposals to make twist-axles incorporating a separatetorsion bar. The torsion bar in this design provides the requiredtorsional stiffness or resistance. In some designs, such as thosedescribed in International Publication No. WO 2006/096980, a torsionelement welded to a cross-beam member replaces the torsion bar toprovide the required torsional resistance. Separate parts allow separatedesign requirements to be met. These proposals, however, requireadditional manufacturing and material costs. There have also beenproposals to manufacture cross-beam members from a tubular blank, suchas those described in U.S. Pat. No. 6,616,157 and U.S. Pat. No.6,487,886. Such a cross-beam member has a mid-section of low torsionalstiffness between two transition sections of high torsional stiffness,to achieve the overall torsional stiffness requirement. The mid-sectionhas a U-shaped, V-shaped, or star-shaped double-walled cross-sectionalprofile of low torsional stiffness. However, as a twisting force isapplied to such a cross-beam member, stresses tend to concentrate in thetransition zones located between the mid-section and the end sections,which may cause durability concerns. Proposals have been made, such asthat taught in U.S. Pat. No. 6,758,921, to selectively heat treat thetransition zones in order to impart desired physical properties to thetransition zones to prevent cracking. This approach, however, introducesadditional manufacturing steps and also requires additional heattreatment equipment.

It is an object of the present invention to mitigate or obviate at leastone of the above mentioned disadvantages.

SUMMARY OF INVENTION

The present invention is directed to a cross-beam member for use in atwist-axle and a method of making the cross-beam member. A broad aspectof the present invention involves a cross-beam member of variable wallthickness, wherein the wall thickness varies along the length of thecross-beam member to meet anticipated local stress requirements andoverall torsional stiffness requirements.

In one embodiment, the cross-beam member has two connection regions anda central mid-section between the two connection regions. The centralportion is torsionally elastic and the connection regions aretorsionally stiff. The connection regions are where the trailing armsare rigidly secured or linked to the cross-beam member. The tubularcross-beam member has a wall thickness varying longitudinally from thetorsionally elastic central portion to each of the torsionally stiffconnection regions. In one feature of the invention, the wall thicknessis larger in at least a portion of the connection regions than in thecentral section. In another feature of the invention, the wall thicknessvaries smoothly along the cross-beam member from the torsionally elasticcentral portion to each of the torsionally stiff connection regions.

In another embodiment, there is a twist-axle that has a cross-beammember with a variable wall thickness. The cross-beam member is aunitary piece and is formed from a tubular blank. The cross-beam memberhas a torsionally elastic central portion and two torsionally stiffconnection regions. The wall thickness of the cross-beam member varieslongitudinally along the length of the cross-beam member from thetorsionally elastic central portion to each of the torsionally stiffconnection regions. In one feature of this embodiment, the cross-beammember has a general U-shape and comprises two trailing arms eachintegrally formed with and extending from one of the connection regionsin a direction transverse to the direction defined by the centralsection. The terminal end of each trailing arm is adapted for a wheel tobe attached thereto. In another feature of this embodiment, thetwist-axle has two trailing arms secured rigidly to the opposite ends ofthe cross-beam member. One end of the trailing arm is adapted forconnecting to the frame of a vehicle and the other end of the trailingarm is adapted for a wheel to be connected thereto.

In other aspects, the invention provides various combinations andsubsets of the aspects described above.

BRIEF DESCRIPTION OF DRAWINGS

For the purposes of description, but not of limitation, the foregoingand other aspects of the invention are explained in greater detail withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a twist-axle that includes a cross-beammember according to an embodiment of the present invention;

FIG. 2 shows in a perspective view a cross-beam member used in thetwist-axle shown in FIG. 1;

FIG. 3 is a cross-sectional view of the cross-beam member of FIG. 2taken along line 3-3.

FIG. 4 is a cross-sectional view of the cross-beam member of FIG. 2taken along line 4-4;

FIG. 5 is a cross-sectional view of the cross-beam member of FIG. 2taken along line 5-5;

FIG. 6 is a cross-sectional view of the cross-beam member of FIG. 2taken along line 6-6;

FIG. 7A illustrates in longitudinal cross-sectional view a tubular blankfor making a cross-beam member shown in FIG. 2;

FIG. 7B illustrates an initial tubular blank of constant wall thicknessthat can be used to form the tubular blank shown in FIG. 7A;

FIG. 7C illustrates a partially flattened tubular blank formed from thetubular blank shown in FIG. 7A;

FIG. 8A shows an example of a longitudinal profile of wall thickness(only one half is shown; the other half is a mirror image thereof);

FIG. 8B shows another example of a longitudinal profile of wallthickness of the cross-beam member shown in FIG. 2 that has a transitionsection divided into three design zones (only one half is shown; theother half is a mirror image thereof);

FIG. 8C shows yet another example of a longitudinal profile of wallthickness of the cross-beam member shown in FIG. 2 that has a transitionsection having a tapered wall thickness (only one half is shown; theother half is a mirror image thereof);

FIG. 8D shows a further example of a longitudinal profile of wallthickness with a tapered wall thickness along the entire half of across-beam member (only one half is shown; the other half is a mirrorimage thereof);

FIG. 9A is a top plan view of a cross-beam member as an example of analternative embodiment to that shown in FIG. 2;

FIG. 9B illustrates an example of a longitudinal profile of wallthickness (only one half is shown; the other half is a mirror imagethereof) of the cross-beam member shown in FIG. 9A; and

FIG. 9C shows steps of a process for producing a cross-beam member shownin FIG. 9A.

DETAILED DESCRIPTION OF EMBODIMENTS

The description which follows and the embodiments described therein areprovided by way of illustration of an example, or examples, ofparticular embodiments of the principles of the present invention. Theseexamples are provided for the purposes of explanation, and notlimitation, of those principles and of the invention. In the descriptionwhich follows, like parts are marked throughout the specification andthe drawings with the same respective reference numerals.

FIG. 1 illustrates a twist-axle 100 in a suspension structure, inparticular, a rear suspension structure. The twist-axle 100 includes across-beam member 102. The cross-beam member is generally elongated,having two opposite ends 104. The twist-axle 100 is typically providedwith two side trailing arms 106.

Each trailing arm 106 has a first end 108 and a second end 110 asillustrated in FIG. 1. The first end 108 is adapted to be pivotallyconnected to a vehicle's frame (not shown) through, for example,connection fitting 112. Each of the trailing arms 106 has a wheel mount114 secured thereto adjacent the second end 110 for supporting a roadwheel (not shown). Spring seat 116, or other support structure forsupporting suspension components or other attachments, may also besecured to the trailing arm 106, and/or to the cross-beam member 102.Each of the trailing arms 106 is rigidly secured to the cross-beammember 102 by welding, bolting, or any other suitable means. The regionconnecting the trailing arm to the cross-beam member 102 is a connectionregion 118, and in this case, is at an end 104 of the cross-beam member102.

When a vehicle moves along an uneven road surface, its wheels tend tomove up and down following the road surface. When the wheels ondifferent sides of the vehicle move up and down by different amountsrelative to the vehicle's body, the unequal vertical displacements ofthe wheels cause the two trailing arms 106 to pivot by different angularamounts. As each end 104 of the cross-beam member 102 is attached to atrailing arm 106, the pivoting of the trailing arms 106 by differentamounts at the opposite ends of the cross-beam member 102 results inturning the opposite ends by different amounts, therefore a twisting ofthe cross-beam member 102. In response to the twisting, the cross-beammember provides a restoring force due to its inherent torsionalstiffness. Similarly, when a vehicle turns, a centrifugal force actingon the center of gravity of sprung mass of the vehicle causes a shift ofweight from one side of the vehicle to the other side and therefore fromone wheel to the other, which also results in unequal pivoting oftrailing arms due to torsional resistance of the cross-beam member. Itis desirable that the cross-beam member is sufficiently torsionallystiff but not too torsionally stiff to provide good ride comfort andgood tire contact with the road, therefore good controllability.

FIGS. 2-6 illustrate an example of a cross-beam member 102 in isolationand its transverse cross-sectional shapes at several selected locations.As noted, the cross-beam member 102 is generally elongated, having twoopposite ends 104, which define a longitudinal direction. The cross-beammember 102 has a central portion, i.e., mid-section 202, and two endportions. Each end portion includes an end section 204 formed at one ofthe opposite ends 104 and a transition section 206 formed between theend section 204 and the mid-section 202. The mid-section 202 istorsionally elastic, which provides the required torsional resistance.The end sections 204 in this embodiment arc the connection regions 118and are torsionally stiff. The transition sections 206 provides atransition from the torsionally elastic mid-section to the torsionallystiff end sections. As will be discussed below, the cross-beam member102 is preferably formed from a tubular blank whereby the mid-section202, the transition sections 206 and the end sections 204 comprise aunitary body. The end sections 204 are preferably adapted to be attachedto side trailing arms 106.

The transverse cross-sectional shape of the cross-beam member 102, i.e.,the cross-sectional shape in a cross-section transverse to thelongitudinal direction, varies along the length of the cross-sectionalmember. The transverse cross-section of mid-section 202 has a generallyopen profile, i.e., a profile that has at least two legs, the legs beingjoined or at least connected at one end and extending generallytransverse of the longitudinal direction so that the other ends arespaced from each other to form the open profile. Some examples of such agenerally open profile include a U-profile, a V-profile, a C-profile, anX-profile or a general, star-shaped profile. Such a generally openprofile allows the mid-section 202 to be torsionally elastic, as thetwisting and bending of legs along a longitudinal direction caused by atorque applied to the cross-beam member allow an elastic shape change ofthe legs and subsequent spring back when the torque is removed. Thetorsional elasticity or stiffness of such a mid-section may be adjusted,for example by adjusting the length of the mid-section having thegenerally open transverse profile, the cross-sectional shape, or thewall thickness of the cross-beam member in the mid-section. Any othertransverse cross-sectional profiles that are suitable for providing atorsionally elastic mid-section may also be selected.

The cross-sectional profile 210 of the cross-beam member illustrated inFIGS. 3 and 4 has a general U-shape. The U-shaped transversecross-sectional profile 210 has two legs 212 and a central connectionportion 214 joining the legs. The transverse cross-sectional profile inmid-section 202 has the form of a flattened loop. Such a profile may beobtained by flattening a portion of a tubular blank and further formingthe flattened portion into a U-shape. This may be a two-step process,i.e., flattening and then shaping, or a combined one-step formingprocess.

As can be seen in FIG. 6, the end section 204 has an end cross-sectionalshape that may be circular, oval, or some other non-circular shape. Sucha shape is suitable for attaching the end section to a side trailingarm. Such a transverse cross-sectional shape also provides a torsionallystiff end section, which as noted earlier, is a connection region. Thetransverse cross-sectional shape of the transition section 206transitions from that of the mid-section 202 to that of the end section204. An example is shown in FIG. 5. Preferably, such transition issmooth and gradual. When the cross-beam member is twisted by oppositetorsional forces exerted on opposite ends 104, the transition sectionstransmit the torsional forces to the mid-section. Smooth transitionhelps avoiding any concentrated build-up of stress in the transitionsection when the mid-section is twisted by the twisting forces exertedat the end sections and transmitted through the transition sections.

The cross-sectional shape in the transition section transitions from across-sectional shape in the mid-section, for example, a U-shape or aV-shape, to an end cross-sectional shape in the end section, such as anoval shape. Because the transition sections are partially pressedinwardly transverse to the longitudinal direction, the shape change inthe transition sections also imparts some torsional elasticity to thetransition sections. The transition section is more torsionally elasticnear the mid-section than near the end section, due to the change in itscross-sectional shape. The cross-sectional shape of the transitionsection and the longitudinal variation of the cross-sectional shape maybe that determined by forming process, for example, by holding the endsections fixed while pressing and forming the mid-section, or may bethat determined by a forming die designed for the transition region,which may provide more precise control of the torsional elasticity andits variation in the transition region. As will also be appreciated, thewall thickness and its longitudinal variation in the transition sectionwill also affect the torsional elasticity and its variation.

Referring to FIG. 2, cross-beam member 102 of the invention ispreferably provided with a varying wall thickness t along its length, asshown in a representative longitudinal profile 222 in FIG. 2. Theexample of cross-beam member 102 has a wall thickness that is generallyuniform circumferentially as can be seen in FIGS. 3 to 6 and varieslongitudinally along the cross-beam member as can be seen in FIG. 2.Typically, the longitudinal profile 222 is generally symmetrical. Thatis, the wall thickness of the cross-beam member varies equally whenmoving from the center of the cross-beam member to either end. However,non-symmetrical longitudinal profiles are also contemplated, forexample, when required to accommodate any non-symmetrical shapes or loadconditions.

FIG. 2 illustrates an example of longitudinal variation of wallthickness, i.e., the variation of wall thickness longitudinally alongthe length of a cross-member beam 102. The mid-section 202 illustratedin FIG. 2 has the thinnest wall, i.e., the thickness is the smallest. Alarger wall thickness of the cross-beam member occurs in the transitionsection 206. In a preferred embodiment, the wall thickness t transitionssmoothly from one section to the next, as illustrated in the overalllongitudinal profile 222. Similar to smooth transitioning of transversecross-sectional profile in the transition section, smooth transitioningof wall thickness from one wall thickness to the next, or from that ofone section to the other, also helps avoiding concentrated build-up oflocal stress, in particular, in any regions of non-smooth transition.

As described earlier, the cross-beam member must meet compliancerequirements of torsional stiffness. A cross-beam member is also a loadbearing component and must also have the required strength, to carry thestress levels generated by torsional, bending, shear, and axial loads.As noted, the transition sections transmit torsional forces exerted onopposite ends to the mid-section. The transition in cross-sectionalshape may cause stress concentration in the transition sections when thecross-beam member is twisted. Durability tends to be affected by anypotential cracks in regions of high stress in the transition sectionscaused by frequent twisting, which is another concern. As can beappreciated, larger thickness allows to reduce stresses in any givenstructure, but it also proportionally increases stiffness. Instead ofselecting a constant wall thickness for the cross-beam member that willbe a compromise between the low stiffness and the maximum allowablestress requirements, the wall thickness and its variation along thelength of the cross-beam member are “tuned”. In other words, the wallthickness and its longitudinal variation are adjusted according todesign requirements such as the overall load bearing and torsionalstiffness requirements, and anticipated local stress concentration. Thevariation of wall thickness is selected to support local stressconcentration. For example, the wall thickness is larger in regionswhere larger stress concentration is expected and smaller where suchlarger stress concentration is not expected. The wall thickness can alsobe reduced where a region is required to be more compliant. Variation ofwall thickness also may be selected to minimize local stressconcentration, which results in a more evenly distributed local stress.Evenly distributed local stress, especially when under severe loadconditions, helps extending service life of the component, as lessstress concentration leads to less early failures in these high stressregions.

As will be appreciated, any one of the mid-section, the transitionsections and the end sections of a cross-beam member can be “tuned” andis often tuned in order to optimize the distribution of mass along thelength of the cross-beam member, while meeting the design requirements,such as local stress distribution, overall torsional stiffnesscompliance, etc. For example, when required by load bearingrequirements, the mid-section may have a wall thickness larger than thatin the transition sections, in the end sections or in both sections, orthe mid-section may have a wall thickness about the same as in one ofthe other sections. Similarly, the other sections may also have largeror smaller wall thickness as required. Any two of the sections, forexample, the end sections and the transition sections, also may have thesame wall thickness. In addition, dividing the cross-beam member into amid-section, two transition sections and two end sections and treatingeach section to have a generally uniform wall thickness are only forconvenience of description. Any of these sections can be divided intosubsections which may have a variable wall thickness within the section,if desirable or necessary.

In general, the wall thickness of the cross-beam member is variedlongitudinally as required. For example, each section may itself have avariable wall thickness. The variation of wall thickness of eachsections and within each section is tuned, i.e., adjusted, according toanticipated local stress, subject to additional factors such as overallrequirements of torsional stiffness, load bearing requirements, materialselected, overall sizes of the cross-beam member and length of eachsections, durability requirements, among others. The longitudinalprofile shown in FIG. 2 is only one example. It will also be appreciatedthat cross-beam member 102 may have other shapes, not limited to thatshown in FIG. 2. Changing the shape of the cross-beam member may alsolead to different local stress distribution and overall torsionalstiffness and load bearing abilities, which may also lead to a differentlongitudinal variation of wall thickness.

A cross-beam member having variable wall thickness as shown in FIG. 2may be formed from a tubular blank 700 having a variable inner diameterand a constant outer diameter as shown in FIG. 7, as will be describedbelow. The tubular blank 700 itself that has variable wall thickness maybe formed using any suitable technique, such as that described in PCTApplication No. PCT/CA2002/00464, the entire content of which isincorporated herein by reference. Briefly, a tubular blank 700 having auniform outer diameter and variable wall thickness is formed from aninitial tube 720 of constant wall thickness (FIG. 7B) using areciprocating mandrel and die assembly through a cold forming process.The die has a die cavity that has an opening corresponding to the outerdiameter of the tubular blank 700. The mandrel has sections of differentdiameters or may be tapered. When cold forming the tubular blank, themandrel is placed inside the tube and selectively moved into or out ofthe opening of the die, or with sections of different diametersselectively placed in the die opening. The die opening has a sizesmaller than the initial outer diameter of the initial tube. The initialtube 720 is drawn through the die opening. As the initial tube is forcedthrough the die opening, the outer diameter of the formed tube isreduced to the size of the die opening. The wall of the tube passingthrough the die is constricted at desired locations by the mandrel andthe die opening, thereby restricting the wall to a thickness defined bythe gap between a section of the mandrel placed in the die opening andthe die opening itself. If the mandrel is removed from the die opening,such constriction is not possible and the wall thickness is unaffectedby the mandrel. By selectively moving the mandrel in and out of the dieopening and selectively placing sections of the mandrel of differentdiameters in the die opening as the tube is drawn through the dieopening, a tubular blank of varying wall thickness is obtained. Afterthe tubular blank reaches the desired or designed length, the tube iscut or severed from the initial tube.

For example, when an initial tube 720 is first drawn through the dieopening, the section of the mandrel placed in the die opening has adiameter such that the difference between the mandrel diameter and dieopening's diameter is twice the wall thickness of the end section inorder to form a end section with the desired wall thickness. After adesired length of the end section is formed, a different region of themandrel is gradually moved into the die opening to form the transitionsection. The difference between of the mandrel diameter in this regionand the die opening diameter is twice the wall thickness of thetransition section. As the repositioning of the mandrel is gradual, theresulting change of wall thickness, namely the transition from that ofthe end section to that of the transition section, also tends to besmooth. After the desired length of the transition section is formed,another different region of the mandrel is gradually moved into the dieopening. The difference between the die opening diameter and the mandreldiameter in this region is twice the wall thickness of the mid-section.After the mid-section is formed, the mandrel is repositioned again toform the second transition section, after which, repositioned again toform the second end section. The tube is then cut to obtain a tubularblank that has a variable wall thickness corresponding to that of thecross-beam member.

When a tubular blank of varying wall thickness is cold formed this wayfrom an initial tubular blank of uniform wall thickness, the coldforming process often introduces stress in deformed regions such thatthe cold-formed tubular blank may become too stiff or too brittle forfurther processing. Preferably, the cold-formed tubular blank is stressrelieved prior to further forming of the tubular blank into a cross-beammember.

As will be appreciated, although a tubular blank 700 shown in FIG. 7Ahas a uniform outer diameter, using such a tubular blank is forconvenience only. In particular, it is for the convenience ofmanufacturing tubular blanks using a die set of fixed die opening. Othertypes of die set and other forming techniques can be employed to producetubular blank 700. Tubular blanks therefore may have variable wallthickness that is due to variation in inner diameters, due to variationin outer diameters or a combination of variations in inner and outerdiameters. For example, the tubular blank shown in FIG. 7A has aninternal tube diameter 710 that varies along the length of the tubularblank 700 and a constant outer diameter 712. The distance between theinner and outer diameters is the wall thickness. As the differencevaries along the length of the tubular blank, the wall thickness variesaccordingly. In the example shown in FIG. 7A, the variation in wallthickness of the tubular blank, and therefore the variation in wallthickness of the cross-beam member formed from the tubular blank, is dueto variation in the inner diameter alone, with the outer diameterremaining generally constant. It is also possible to keep inner tubediameter 710 constant and vary the outer diameter 712 along the lengthof the tubular blank. The variation in wall thickness will then be dueto the variation in the outer diameter alone. Of course, both inner andouter diameters may vary along the length of the tubular blank and cancontribute to the variation of wall thickness along the tubular blank,and therefore the variation of wall thickness of the cross-beam member.

FIG. 7A shows an example of a tubular blank 700 that has a longitudinalprofile of wall thickness 702 corresponding to that of the cross-beammember shown in FIG. 2. The tubular blank shown in FIG. 7A has twoopposite end regions 706 corresponding to the end sections 204, twointermediate, transition regions 708 formed between the end regions 706and a central region 704 formed between the transition regions 708. Thetransition regions 708 correspond to the transition section 206 and thecentral region 704 corresponds to the mid-section 202. Other than somepossible small changes in wall thickness caused by a forming processdescribed below, the wall thickness of the end regions is essentiallythe same as the wall thickness in the end sections 204 of the cross-beammember, the wall thickness in the transition regions 708 is essentiallythe same as the wall thickness in the transition sections 206 and thewall thickness in the central region 704 is essentially the same as thewall thickness in the mid-section of the cross-beam member. After such atubular blank 700 is obtained, the tubular blank is deformed, e.g.,press formed, to obtain a cross-beam member.

To form a cross-beam member 102, the tubular blank 700 may be firstflattened in a substantial portion in the middle and further deformedinto the U-shaped cross-sectional profile in the central region 704.Forming the central region 704 into the U-shaped profile may be atwo-step process, for example. In a two-step process, the first step isto flatten the central region, a substantial portion of the centralregion, or the central region and part of the neighboring transitionregions, to obtain a partially flattened tubular blank 730, asillustrated in FIG. 7C. The flattened portion 732 of the partiallyflattened tubular blank 730 is next bent to form the U-shapedcross-sectional profile. Of course, these two steps, i.e., flatteningand bending, can also be carried out in a combined one-step process. Forexample, the tubular blank 700 may be placed in a forming die which hasa longitudinal U-shaped surface and then have a substantial portion ofthe tubular blank flattened and deformed at the same time to conformwith the U-shaped surface of the forming die. As the central region 704is deformed, e.g., shaped by a forming die or pressed and bent, theintermediate, transition region 708 are deformed by forces exerted bythe central region 704 that is being deformed. The cross-sectionalprofile of the cross-beam member preferably transitions smoothly fromone end section, through transition sections and the mid-section, to theother end section. A cross-beam member 102 having a longitudinal profile222 of wall thickness and a cross-sectional profile that transitionsfrom a general U-shape in the central region to a generally flattenedoval shape near the ends can be formed from a tubular blank.

As noted, none of the end sections, transition sections and themid-section in general need to have a constant wall thickness. Any ofthem may have regions of different wall thicknesses to meet the designrequirements for these sections. FIG. 8A shows an example of alongitudinal profile representing a cross-beam member that has itstransition section divided into two regions, the region adjacent the endsection having a larger wall thickness while the wall thickness in theother region is smaller. FIG. 8B shows another example, in which thetransition section 206 is divided into three zones, namely zone 1, zone2 and zone 3, zone 1 being adjacent the end section 204 and zone 3 beingadjacent the mid-section 202. Zone 2 is formed between zone 1 and zone3. Each of these zones may be tuned, i.e., with its wall thicknessadjusted according to design requirements, and are referred to as designzones. As one example, the wall thickness in zone 1 may be larger thanthat in zone 2, which may be larger than that in zone 3, which may be inturn larger than that in the mid-section. As another example, zone 3 mayhave the smallest wall thickness, with zone 2 having the largest wallthickness and the mid-section having a wall thickness between that ofzone 2 and zone 3. Of course, different number of design zones in eachsection, other distributions of wall thicknesses in these design zonesand their values in relation to wall thicknesses in the mid-section andthe end sections are also possible, depending on specific designrequirements and constraints for different specific vehicles. FIG. 8Cillustrates another example of variation of wall thickness. The wallthickness of end section 204 is larger than the wall thickness ofmid-section 202. The transition section 206 between the end section 204and the mid-section 202 has a tapered wall thickness, i.e., the wallthickness in the transition section decreases continuously toward themid-section. FIG. 8D provides yet another example in which the wallthickness decreases continuously toward the middle of the cross-beammember in all three sections, namely end section 204, transitionssection 206 and mid-section 202.

FIG. 9A illustrates an example of another embodiment of a twist-axle100′. Instead of a generally straight cross-beam member, twist-axle 100′has a generally U-shaped cross-beam member 102′. The U-shaped cross-beammember 102′ has a generally straight mid-section 202 and two transitionsections 206, with two integrated trailing arms 120 forming the legs ofthe U. Each integrated trailing arm 120 extends from a connection region118′ of the U-shaped cross-beam member 102′. Each integrated trailingarm 120 has a terminal end 122, which is adapted for a wheel to beconnected thereto, such as having a wheel mount 114 secured thereto. TheU-shaped cross-beam member 102′, including the mid-section 202, thetransition sections 206, the connection regions 118′, and the integrallyformed trailing arms 120, is a unitary piece and is formed from onetubular blank, as will be described in detail below. The general shapeand transverse cross-sectional profile of the mid-section 202, thetransition sections 206, the connection regions 118′ are substantiallythe same as that of the general straight cross-beam member 102 oftwist-axle 100, other than the bends in the connection regions 118, andtherefore will not be described in detail here.

The twist-axel 100′ also has a pair of side arms 124, which correspondwith the front portion of the trailing arms 106 of twist-axle 100 shownin FIG. 2. Each side arm 124 has one end adapted for securing to theconnection region of the cross-beam member 102′. This may be, forexample, by way of welding, bolting, or some other suitable means. Inthe example shown in FIG. 9A, side arm 124 is welded to spring seat 116and to cross-beam member 102′ in the connection region 118′. Each of theside arms 124 has its other end adapted to be connected to a vehicle'sframe through, for example, connection fitting 112. The side arms 124may be tubular or they can be stamped. They also can have either open orclosed cross-sectional shapes.

The cross-beam member 102′ has a variable wall thickness varying alongthe length. The variation of wall thickness provides a torsionallyelastic central section and torsionally stiff connection regions. Atleast, a portion of each connection region 118′ where integrally formedtrailing arm 106 is formed is torsionally stiff. The wall thicknessgenerally varies smoothly along the cross-beam member from one terminalend 122 to the other terminal end 122.

FIG. 9B shows an exemplary longitudinal profile of wall thickness ofcross-beam member 102′ (only one half is shown, the other half is amirror image thereof). The wall thickness is about 2.7 mm in theintegrated trailing arm 120 section and then is increased to about 3.4mm in the connection region 118′. The wall thickness is the smallest inthe central section 202 in this example, about 1.7 mm. The example ofthe cross-beam member 102′ shown in FIG. 9B also has a transitionsection 206 formed between each of the connection regions 118′ and thecentral section 202. The transition section 206 in this example has awall thickness between that of the connection region and that of thecentral section and is about 2.3 mm. Of course, it will be appreciatedthat the relative wall thicknesses and their values in different regionsin this example are for illustration only and may be different dependingon specific design requirements and constraints for different specificvehicles.

To form a cross-beam member 102′, a series of steps typically will berequired. FIG. 9C illustrates steps of a process, which also includes anumber of optional steps, for forming a cross-beam member 102′. Theprocess 900 starts with forming (step 910) a tubular blank 700 that hasa variable wall thickness. The details of forming a tubular blank ofvariable wall thickness have been provided in connection with forming across-beam member 102 and will not be repeated here. The tubular blank700 has a longitudinal profile of wall thickness corresponding to thatof cross-beam member 102′, one example of which is shown in FIG. 9B. Thetubular blank 700 is next stress relieved (step 912) in regions wheresignificant deformation is expected, such as regions corresponding tocentral section and connection regions of the cross-beam member. Nextand optionally, in a pre-bend step 914, the stress relieved tubularblank is bent in the connection regions to shape the stress relievedtubular blank into a “U”. The central section is next formed at step 916to form the generally open transverse profile, in the manner describedearlier in connection with forming a generally straight cross-beammember 102, which will not be repeated here. Next, at step 918, theintegrated trailing arms 120 are formed. The integrated trailing arms120 formed at this step may be further sized where necessary. Finallyand optionally, heating and quenching may be applied (step 920) in areaswhere higher strength is required or desirable, such as in theconnection regions or the transition regions and shot peening may befurther applied (step 922) in these areas. Conveniently or preferably,heating and quenching may be applied to the entire cross-beam member102′. Likewise, both shot peening partial or full surfaces arecontemplated. Shot peening also can be applied on inner surface, outersurface or both inner and outer surfaces of the tubular cross-beammember 102′.

As noted, some of the steps described above are optional. For example,depending on specific applications or production requirements, heatingand quenching (step 920) and the immediately followed shot peening step922 may not be necessary. In addition, as will be appreciated, some ofthe steps may not necessarily follow the order illustrated anddescribed. For example, heating and quenching (step 920) and thesubsequent shot peening step 922 may also be carried out prior to thepre-bending step (step 914), again, depending on design requirements.

It will be appreciated that wall thickness, for any given loadrequirement and torsional stiffness requirement, is affected by materialselected. One material suitable for making cross-beam members is an HSLAsteel, such as an HSLA8OF steel (YS 80ksi, UTS 95ksi, 20% UniformElongation). An HSLA steel is generally preferred as it providesrequired high strength for some typical applications, without requiringsubsequent quench and temper operations after the cross-beam member isformed. Although HSLA steel is preferred, other materials may be used.For example, while heat treatment is preferably to be avoided, it isalso contemplated that, in order to reduce weight further or to meetparticularly low values of stiffness, other materials having even higherstrength but requiring heat treatment may be used. One such material isboron steel. Boron steel, because of its considerably higher strength,can more easily meet the load requirements than HSLA steel but with lessweight or with lower stiffness of the axle. A cross-beam member may bemade from a boron steel, such as Mn22B5 steel. However, heat treatmentof the transition sections generally will be required in order to hardenthe heat treated regions to achieve desired yield point. The transitionsections can be heat treated before or after the mid-section is formedinto the U-shaped cross-sectional profile. Heat treatment is alsocontemplated where higher strength is required in certain specialregions. One such example is provided above, in connection withdescribing forming a U-shaped cross-beam member.

Various embodiments of the invention have now been described in detail.Those skilled in the art will appreciate that numerous modifications,adaptations and variations may be made to the embodiments withoutdeparting from the scope of the invention. Since changes in and oradditions to the above-described best mode may be made without departingfrom the nature, spirit or scope of the invention, the invention is notto be limited to those details but only by the appended claims.

What is claimed is:
 1. A method of making a unitary tubular cross-beammember for use in a twist-axle of a vehicle, the cross-beam memberhaving two connection regions defining a longitudinal direction and acentral section formed between the connection regions, the centralsection being torsionally elastic and having a generally open transverseprofile, the connection regions being torsionally stiff in at least aportion thereof, the cross-beam member having a wall thickness that isgenerally uniform circumferentially and varying longitudinally from thetorsionally elastic central section to each of the torsionally stiffconnection regions, the method comprising: a) cold forming an initialtubular blank of uniform wall thickness to obtain a tubular blank ofvariable wall thickness, the tubular blank having a central regioncorresponding to the central section, the variable wall thickness of thetubular blank corresponding to the wall thickness of the cross-beammember, and b) deforming the central region of the tubular blank toconform with the generally open transverse profile of the centralsection to obtain the cross-beam member.
 2. The method of claim 1,wherein step a) includes providing gradual transition of the wallthickness from the central section to the connection regions.
 3. Themethod of claim 1, wherein the cross-beam member has two opposite endsand each of the connection regions is formed at one of the oppositeends, the cross-beam member further comprising an end section formed ateach of the opposite ends of the cross-beam member and a transitionsections formed between the end section and the central section, step b)further comprises: deforming the central region while keeping an endcross sectional profile of each of the end sections unchanged andallowing a transverse cross-sectional profile of the cross-beam memberto vary gradually in each of the transition sections.
 4. The method ofclaim 1, wherein the cross-beam member has a transverse cross-sectionalprofile varying longitudinally along the length of the cross-beammember, the transverse cross-sectional profile having a U-shape orV-shape in the central section, wherein step b) includes: flattening thecentral region corresponding to the central section to obtain apartially flattened tubular blank, and bending the flattened region toform the U-shaped or V shaped transverse profile, wherein the transversecross-sectional profile of the cross-beam member is allowed to varysmoothly from the U-shape or V-shape in the central section to an endcross-sectional profile of the connection regions.
 5. The method ofclaim 1, wherein step a) includes: providing an initial tubular blank ofuniform wall thickness and an initial outer diameter, drawing theinitial tubular blank through a mandrel and die assembly, said mandreland die assembly including a die with a die opening smaller than theinitial outer diameter and a mandrel having regions of differentdiameters, wherein the mandrel is selectively placed in the die openingas the initial tubular blank is drawn through the die opening to obtainvariable wall thickness of the tubular blank.
 6. The method of claim 1,further comprising: releasing stress in the tubular blank of variablewall thickness prior to step b).
 7. The method of claim 1, wherein thecross-beam member has a general U-shape and comprises two trailing armseach integrally formed with and extending from one of the connectionregions in a direction transverse to a longitudinal direction defined bythe connection regions, the method further comprising: c) bending thecross-beam member in each of the connection regions to form theintegrated trailing arms.
 8. The method of claim 7, further comprising:b1) prior to step b), pre-bending the tubular blank to obtain a U-shapedpre-deformed tubular blank.
 9. The method of claim 7, furthercomprising: d) subsequent to step c) or prior to step b), applyingheating and quenching to the cross-beam member.
 10. The method of claim9, wherein the heating and quenching are applied in the connectionregions.
 11. The method of claim 9, further comprising: e) immediatelysubsequent to step d), applying shot peening to the cross-beam member.12. The method of claim 11, wherein the shot peening is applied in theconnection regions.